The basic project objective is to determine the local ordering of histones and other proteins along chromatin fibrils and the structural and functional consequences of this protein ordering. Recent research in this laboratory and elsewhere has demonstrated that chromatin fibrils consist predominantly of "subunits" containing 140 base pairs of DNA tightly associated with and compactly folded by two copies each of histones H2A, H2B, H3 and H4. These units are joined in tandem by short (30-60 base pairs), extensible, nuclease-susceptible regions of "spacer" DNA. Cleavage of chromatin by nucleases occurs between subunits and at specific sites within subunits, yielding submonomer, monomer and oligomer nucleoprotein units that can be readily isolated by sedimentation on sucrose gradients. Current research entails characterization of isolated submonomer and monomer units in terms of composition, appearance in the electron microscope, physical properties and accessibility to specific chemical reagents that react with DNA or proteins. Such studies should provide information about the linear arrangement of histones along DNA and the overall conformation of the subunits. Efforts are also in progress to isolate and characterize other functionally distinctive regions of chromosomes such as regions active in gene expression and centromeric regions rich in highly redundant DNA. BIBLIOGRAPHIC REFERENCE: Darlene K. Oosterhof, John C. Hozier and Randolph L. Rill, "Nuclease Action on Chromatin: Evidence for Discrete, Repeated Nucleoprotein Units Along Chromatin Fibrils." Proc. Nat. Acad. Sci. USA 72, 633-637 (1975). R. L. Rill, D. K. Oosterhof, J. C. Hozier and D. A. Nelson, "Heterogeneity of Chromatin Fragments Produced by Micrococcal Nuclease Action." Nucleic Acids Research 2, 1525-1538 (1975).